The invention relates to a glazing panel comprising a glass substrate provided with a functional thin film, the latter exhibiting properties of transparency, electrical conductivity and/or low emissivity.
It also relates to the processes for obtaining such a glazing panel, in particular by means of pyrolysis or of techniques-using a vacuum.
This type of functional film is most particularly applicable for glazing panels intended for buildings: a glass substrate, coated with a low-emissivity film, makes it possible to reduce the emission in the far infrared through the glazing panel, of which it forms part, from inside the premises to the outside. By reducing the energy losses due partly to this radiation leakage, the comfort of the inhabitants is appreciably improved, especially in winter. The substrate thus coated and then combined with another substrate via a layer of gas, the low-emissivity film lying as face 3 (counting from the outermost face), constitutes a very efficient insulating double glazing.
These films are also applicable for glazing panels intended for motor vehicles because of their electrical conduction properties, for example in order to form heated windows by providing current leads.
Films of metal oxides exhibiting these properties are, for example, layers of tin-doped indium oxide (ITO), or aluminium-doped zinc oxide (Al:ZnO), indium-doped zinc oxide (In:ZnO), tin-doped zinc oxide (Sn:ZnO) or fluorine-doped zinc oxide (F:ZnO), or fluorine-doped tin oxide (F:SnO.sub.2).
These metal oxide films may be obtained by various processes: by vacuum processes (thermal evaporation or sputtering, possibly using a magnetron) or by pyrolysis of organometallic compounds sprayed in liquid, solid or gas form by a carrier gas onto the surface of the glass substrate heated to a high temperature, but nevertheless one which is less than its softening temperature. These compounds, thus brought into contact with a hot surface, decompose, oxidizing to form a film of metal oxide on this surface. The latter technique is most particularly advantageous insofar as it makes it possible to envisage continuous deposition directly on the ribbon of glass in a float-type manufacturing line.
However, in order for these films to reach a high performance level, especially in terms of emissivity and/or electrical conduction values, their thickness must be at least 180 nm or indeed greater than 400 nm, and usually between 350 and 550 nm. A thin film having such thicknesses gives the substrate which it covers an appearance in reflection, especially a coloration, which may not be highly appreciated from an aesthetic standpoint.
Thus arises the problem of the appearance in reflection, most particularly of the residual coloration obtained, of the substrate coated in this way. This is because, according to the teaching of the patent EP-B-0,125,153, a film of fluorine-doped tin oxide F:SnO.sub.2, the thickness of which is only approximately 163 to 165 nm, deposited on a clear float glass substrate 4 mm thick gives the latter a coloration in reflection in the blues, which coloration is currently greatly appreciated both in the building industry and in the motor-vehicle industry. On the other hand, it has been observed that a film of the same kind but this time having a thickness of 360 nm, and therefore a film having superior performance, gives the same substrate an appearance in reflection, "on the film side", in the greens/purples, which colour is not very desirable for aesthetic reasons.
Various technical solutions intended to improve the optical appearance of glazing panels provided with functional films having such characteristics have already been proposed. Thus, according to the teaching of the European Patent Application EP-A-0,573,325, a so-called "intermediate" coating is sandwiched between substrate and functional film, the optical thickness of which coating has been suitably selected so as to obtain a residual colour in reflection which is in the blues and is, additionally, not very intense. With the same purpose in mind, Patent Application WO-94/25410 proposes to place the functional film this time between two coatings, one called the "internal" coating and the other called the "external" coating, the combination of the choice of the thickness characteristics and of the refractive index of these two coatings widening the scope for finely adjusting the colorimetric appearance of the glazing panel in reflection.